1. Field of the Invention
The present invention relates to a zoom lens system, and more particularly to a compact and high-zoom-ratio zoom lens system suitable for use in digital still cameras.
2. Description of the Prior Art
In recent years, as personal computers become prevalent, digital still cameras have been coming into wider and wider use that allow easy shooting of images. This trend has created an increasing demand for more compact and more low-cost digital still cameras that offer higher zoom ratios and higher optical performance. This in turn has created an increasing demand for more compact and more low-cost taking optical systems that offer higher zoom ratios and higher optical performance. Moreover, higher and higher image quality has been sought after in digital still cameras. In digital still cameras, image quality depends on the number of pixels that the solid-state imaging device provided therein has. For example, a now typical solid-state imaging device of the so-called VGA class has about 330,000 pixels.
As a taking optical system of the VGA class, it is possible to employ an optical system designed for use in a home-use video camera, because it offers a high zoom ratio at low cost. However, with a VGA class taking optical system, it is impossible to obtain satisfactorily high image quality, because the image quality it offers is far inferior to that obtained with a silver-halide film camera. To obtain satisfactorily high image quality, it is necessary to use at least 1,000,000 pixels, and the greater the number of pixels, the higher optical performance is required in the taking optical system. The problem here is that most taking optical systems that offer image quality as high as would be required to cope with 1,000,000 or more pixels are built as fixed-focal-length lens systems. In other words, to obtain a zoom lens system (in particular, a high-zoom-ratio zoom lens system) satisfying such requirements, there is no alternative but to use an interchangeable lens designed for use with a single-lens reflex camera, or a zoom lens designed for use with a professional-use video camera. However, such zoom lenses are extremely large and expensive. Moreover, in a taking optical system, high optical performance is required not only for an object at infinity but also for an object at a close distance, and therefore the focusing method employed therein greatly affects the overall optical performance.